There is a critical need for a stand-alone, low-power, voltage-controlled, non-volatile logic device capable of performing logic functions with low switching and static power dissipation. A need exists for logic devices beyond CMOS that can complement or even replace CMOS technology to sustain exponential growth in chips throughput. Magnetic devices have been at the center of this search as they provide new features such as non-volatility and low-voltage operation.
Existing MEMTJ devices use a magnetoelectric antiferromagnetic (AFM) layer stacked with a magnetic tunnel junction (MTJ) made of a material such as Chromia (Cr2O3). The boundary magnetization of Cr2O3 can be isothermally controlled via an applied electric field. The generated voltage-controlled perpendicular exchange bias can be used to switch an adjacent ferromagnetic layer. The magnetization of the free magnet determines the output MTJ resistance. By using a field-effect transistor (FET) at the output, the MTJ resistance can be converted back to voltage, which drives the next stage. However, exiting MEMTJ logic devices have several drawbacks. First, each device needs multiple dedicated MOSFETs to drive the next stage, which increases the area/delay/energy overheads. Second, a preset and clocking scheme is required to perform logic functions since the output voltage is only positive. Third, the devices are very sensitive to the insulator thickness variability because the output voltage is determined by the voltage division between the FET and the MTJ. Any variation in the insulator thickness changes the MTJ resistance exponentially, therefore, would shift the output voltage significantly.
In sum, existing current-driven magnetic and spintronic devices require high current densities. These high current densities increase power dissipation, cause reliability issues, and lead to static power dissipation. Voltage-controlled magnetoelectric magnetic tunneling junction (MEMTJ) devices are expected to dissipate orders of magnitude less energy per binary switching operation compared to current-controlled magnetic devices. However, existing MEMTJ devices are not concatenable. Instead, connecting multiple MEMTJ devices together requires multiple field effect transistors to reset the devices at the beginning of each clock cycle and to drive subsequent logic stages.
What is needed, therefore, is an improved MEMTJ that allows concatenation, and does not require substantial additional devices to support their operation. Embodiments of the present invention address this need as well as other needs that will become apparent upon reading the description below in conjunction with the drawings.